This invention relates to a protective sheath for a temperature sensing device, as for example a thermocouple, which has a long performance lifetime under conditions in which either continuous or intermittent measurements are made of the temperatures of molten metals.
Many industrial and scientific processes require the measurement and control of extremely high temperatures. For example, measurements of the temperature of molten metals are essential to proper process control in the metal processing industry. Two of the most common instruments used to determine the temperature of molten metals are the optical pyrometer and the disposable lance thermocouple. However, each of these devices has disadvantages. The optical pyrometer is not as accurate as is desirable, and can measure only the surface temperature of the molten metal. The disposable lance thermocouple is inaccurate, does not permit continuous measurement of the temperature of the molten metal, and its use involves some safety problems for the person using it.
As a result of the shortcomings of the optical pyrometer and the disposable lance thermocouple, considerable effort has been expended to develop an immersion pyrometer with a long-term continuous reading capability. In one type of an immersion pyrometer a thermocouple junction is encased in a tube made of a metal with a high melting temperature which is coated with a ceramic, such as Al.sub.2 O.sub.3 or a mixture of Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 which protects the metal tube itself from the molten metal environment. However, in use the ceramic layer or layers tended to spall as a result of large cracks caused by differences in coefficients of thermal expansion. This permitted molten metal to contact the metal substrate and attack it. Attempts were made to overcome the spalling tendency by matching as closely as possible the coefficients of thermal expansion of contiguous layers and by densifying the layers. These attempts included the use of small changes in compositions in proceeding from one layer to the next layer (grading), and densifying the layers as by sintering. Commercially useful protective sheaths for thermocouples have been made by these and other methods; however, immersion pyrometers having a longer lifetime are desirable.